Project Summary/Abstract The hematopoietic stem cell (HSC) niche is a specialized microenvironment that supports the life-long self- renewal of HSCs and their ability to produce all blood cell lineages. A number of different cell types and molecular factors have been associated with HSC niches in the mammalian bone marrow. To date, however, the challenges of directly visualizing the bone marrow have precluded a rigorous, systematic investigation of the cell-cell interactions that promote the niche engraftment of HSCs. Zebrafish offer an unparalleled setting in which the dynamic interactions between HSCs and their supporting niche cells can be experimentally manipulated and directly visualized at a resolution and throughput not possible in any other organism. The overarching goal of this proposal is to use live cell imaging in the zebrafish embryo, in combination with new technologies in gene expression analysis and tissue-specific gene disruption, to elucidate novel cellular and molecular mechanisms that are required for the engraftment of HSCs within their niche. The goal of the first aim is to integrate multiple spatial and tissue-specific gene expression datasets to generate a comprehensive map of gene expression within the zebrafish caudal hematopoietic tissue (CHT), a transient HSC niche akin to the mammalian fetal liver. The resulting map of cell-specific gene expression will be used to guide functional investigations of the cell-cell interactions that are required for HSC engraftment within the CHT. This gene expression data will be used to generate a reporter transgene that specifically labels stromal cells in the CHT. In addition, this gene expression data will be used, in combination with a tissue-specific CRISPR system, to identify the cellular adhesion molecules that mediate macrophage-HSC interactions and are required for HSC engraftment in the CHT. The second aim will investigate the effects of oxidative stress on the HSC niche within the CHT. This will be done by treating embryos with compounds that induce oxidative stress in combination with a live cell dye that permits visualization of reactive oxygen species (ROS). Additionally, the function of two genes (sepp1a and nrros), with known roles in maintaining low ROS levels, will be investigated using loss-of- function and overexpression experiments. These studies could identify novel therapeutic targets to enhance the engraftment and maintenance of HSCs during transplantation-based treatment of blood disorders. As a postdoctoral fellow, Dr. Hagedorn will conduct his research in the laboratory of Dr. Leonard Zon, a renowned hematologist and stem cell biologist. Building on a strong background in genetic and live cell imaging, Dr. Hagedorn will expand his expertise to include new technologies in spatial gene expression and tissue-specific gene disruption. Under the guidance of Dr. Zon and an exceptional mentoring committee, Dr. Hagedorn has constructed a rigorous research and training plan that will allow him to succeed in the mentored and independent phases of the award. The environment at Boston Children's Hospital and Harvard Medical School will provide the ideal surroundings for Dr. Hagedorn to become a successful independent scientist.